1. Field of the Invention
This invention relates generally to a system and method for removing nitrogen from the anode side of a fuel cell stack and, more particularly, to a system and method for removing nitrogen from the anode side of a fuel cell stack that includes using both a bleed valve and a purge valve to remove the nitrogen.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
The MEAs are porous and thus allow nitrogen in the air from the cathode side of the stack to permeate therethrough and collect in the anode side, referred to in the industry as nitrogen cross-over. Nitrogen in the anode side of the fuel cell stack dilutes the hydrogen such that if the nitrogen concentration increases beyond a certain percentage, such as 50%, the fuel cell stack becomes unstable and may fail.
It is known in the art to provide a bleed valve at the anode gas output of the fuel cell stack to remove nitrogen from the anode side. The bled hydrogen can be sent to any suitable location, such as a combustor or the cathode input to the stack. It is also known in the art to provide a purge valve at the anode gas output of the fuel cell stack to depressurize the anode side for rapid system shut-down and to reduce the pressure of the anode side if it significantly increases above the pressure of the cathode side. The purge valve is selectively opened because it creates a pressure drop between the cathode side and the anode side that could reduce the stack lifetime. The bleed valve has a much smaller orifice than the purge valve so that opening the bleed valve does not create a significant pressure difference between the anode side and the cathode side, which could reduce the life of the stack.
A control model is used to control the opening of the bleed valve during operation of the fuel cell stack to maintain the concentration of nitrogen below the certain percentage. As the fuel cell stack ages, the cross-over of nitrogen from the cathode side to the anode side increases as a result of degradation of the MEAs. The control model considers the increase of nitrogen cross-over, and controls the bleed valve accordingly to reduce the concentration of nitrogen in the anode side. However, eventually the fuel cell stack will age enough where maintaining the bleed valve in a constant open position will not remove enough of the nitrogen. Therefore, the fuel cell stack may fail as a result of dilution of the hydrogen. It is not desirable to provide a bleed valve having a larger orifice because the pressure drop created between the cathode side and the anode side when the bleed valve is open would also act to reduce the lifetime of the fuel cell stack.